Setting:
Will install a version of fcitx with those addons pre-installed.
Running:
Will put that plugin in a random path and not use it.
I’d start by removing that package again:
nix-env --uninstall nixos.fcitx5-m17n
Next up, let’s look at that missing library. Looking at your addon, it does not look like it depends on m17n at all: nixpkgs/pkgs/tools/inputmethods/fcitx5/fcitx5-chinese-addons.nix at 5c211b47aeadcc178c5320afd4e74c7eed5c389f · NixOS/nixpkgs · GitHub
It’s also not included in the default addons: nixpkgs/pkgs/tools/inputmethods/fcitx5/with-addons.nix at 5c211b47aeadcc178c5320afd4e74c7eed5c389f · NixOS/nixpkgs · GitHub
So where does that come from? Is the config you’re showing really your whole config? Did you forget to nixos-rebuild switch?
It’d also be nice to see what you have installed with nix-env, to see if that might be causing conflicts. Please share the output of nix-env -q.
I actually don’t recommend using nix-env, unless you really know what you’re doing. It can cause issues like this one, though in this case I think it’s unlikely to be the culprit. We’ll see!